A converter is generally used as an energy conversion device, converting energy in a certain format into energy in a different format, for example, converting DC power into AC power. In particular, due to the higher quality of their output waveform and their higher voltage capability, multi-level converters such as three-level converters, five-level converters or converters with higher levels are gaining widespread application in many industries, providing AC output voltage or AC output current for particular loads (e.g., AC electric machines).
For multi-level converters at present, such as multi-level converters with Neutral Point Piloted (NPP) topologies fabricated by nesting two or more modules together (e.g., five-level converters), there is a technical issue: during the startup phase of the load (e.g., AC electric machines), due to the lower modulation index, certain switching devices inside the multi-level converter (including diodes and switching tubes) exhibit greater losses (such as conduction loss) compared to other switching devices, or alternately the thermal conditions between multiple switching devices may be imbalanced, thereby leading to certain switching devices sustaining greater thermal stress. In order to avoid damages to some of the switching devices because they are sustaining greater localized thermal stress, a solution at present is to de-rate the output current during the startup phase. However, this solution is not suitable for situations where full torque output is required, including but not limited to elevators, therefore this solution is unable to maximize the utilization of such multi-level converters' output capacity. Therefore, it is necessary to design a new solution which does not require de-rating the output current under a low modulation index, or that can minimize the de-rating of output current as much as possible, thereby resolving the existing technical difficulties.